There has been a rapid development in the field of digital television broadcasting following the establishment of digital video broadcasting standards for digital terrestrial television (“DVB-T”). Generally, in accordance with a DVB-T standard, a number of carrier frequencies are used to spread data to be transmitted over a large number of orthogonal frequency carriers. Each carrier can be encoded to carry a symbol containing more than one bit, for example using a rectangular constellation modulation system such as 16-QAM, 64-QAM, etc.
FIG. 1 illustrates a 16-QAM constellation map. In a typical constellation map, each point can represent multiple bits. As is known by a person having ordinary skill in the art, each point on the 16-QAM constellation diagram corresponds to a 4-bit symbol, e.g., the four bits can be in a following format b0b1b2b3. Thus, there are 16 distinct combinations of the four bits. The symbols can be assigned to the constellation points according to various coding schemes.
On the transmitter side, the multicarrier signal is modulated in accordance with successive symbols to be transmitted. The multicarrier signal can be received and demodulated on the receiver side to corresponding symbol(s) using a constellation map. In most cases, the signal received will not correspond exactly with a constellation point because of interference or noise in the channel between the transmitter and the receiver. In this situation, the receiver must demodulate the received signal to the symbol corresponding to the constellation point which is most likely to have been transmitted. It is known to de-map signals using soft decision decoding in which instead of a “hard” decision as to whether a bit should be decoded as a “1” or as a “0”, a “soft” decision, comprising the hard decision and an indication of the level of confidence to be placed in the decision is output. The information from the de-mapper is passed to a decoder, e.g., a Viterbi decoder, which decodes the bits.
One problem with television broadcasting is the existence of multi-paths arising either as a result of the reception at the receiver of multiple copies of the signal emitted from a single transmitter, or as a result of the reception of signals from a number of transmitters all broadcasting the same signal. In the frequency domain, the existence of multi-paths is equivalent to a frequency selective channel response. Furthermore, in situations where conventional analog television signals are transmitted within or overlapping the frequency range used by the digital television signal, the conventional analog television signals act as narrow interfering signals within the signal bandwidth of the digital television signal.
These frequency selective channel response characteristics result in a large number of different carriers used in the modulation of the signal having different signal to noise ratios (“SNR”). Clearly, data conveyed by carriers having a high SNR is likely to be more reliable than data conveyed by carriers having a low SNR.
An estimate of the SNR of each carrier made by the receiver is called the channel state information (“CSI”) for the channel represented by that carrier. This method utilizes pilot carriers with known magnitudes. An estimate is made of the mean square error in the magnitude of the received pilot carriers. Also, the channel state information in the pilot carrier positions can be obtained from this estimate. The channel state information for other data positions can be obtained by subsequent interpolation between the values calculated at the pilot carrier frequencies.
In order to provide robust performance of the system in an environment having a frequency selective channel response, it is known to use the channel state information in the Viterbi decoder when decoding the bits in order to provide extra information regarding the reliability of the bits based on the signal to noise ratio of the carrier. Previously it has also been suggested that if the channel state information of a particular channel is sufficiently bad, it can be concluded that no reliance can be placed on the data received on that channel. As a result, the Viterbi decoder may effectively record that no information is available regarding that bit by disregarding, or “puncturing” the corresponding bit or bits.
The transmitted data is coded using a convolutional code, which introduces redundancy in the signal in order to allow error correction of the signal to be achieved. The effect of the puncturing of data bits in the Viterbi decoder as indicated above is merely to reduce the effective code rate of the signal. If a sufficiently robust code is used, the effective reduction in code rate resulting from the puncturing of bits can be tolerated, thus avoiding an impact on the decoded signal quality. Therefore, there is a desire to provide new methods and systems for demapping that take into account cases where the signal to noise ratio can be varying and/or non-flat.